The present invention relates to improvements in electromagnet systems which include a permanent magnet and a parallel connected shunt path of magnetically conductive material.
Electromagnet systems for instance, of the type used in electromagnetic relays, usually have permanent magnets of comparatively large internal resistance, if regarded as sources of magnetic potential. Consequently, the magnetic potential varies at loading and unloading, for example, due to variations of the air gap between the yoke and armature of the magnet. This potential varies also if an external excitation is superimposed for switching the relay. Such a non-constant magnetic potential leads generally to deterioration of the working characteristic of the relay, and in many instances (i.e. over excitation) the relay-characteristic may shift irreversibly.
It is not possible to maintain a constant magnetic potential with conventional means, because permanent magnets have permeability values which correspond nearly to that of air, or are four to five time higher than that of air.
Such magnets possess the quality that their magnetic potential at the polefaces actually drops considerably during loading, i.e. when the armature approaches the pole shoe and/or during superimposing of an excitation flux acting in the same direction as the permanent flux. In other words, the magnetic flux does not increase as nearly proportionally as it could be expected to do. This holds even more for the forces acting on the armature which vary with the square of the magnetic flux value. Such a non-constant magnetic potential is utilized deliberately in some known relay designs in order to obtain a small pick-up excitation in the working airgap and a linear characteristic for the force of the permanent magnet as acting on the armature. However, the sensitivity of the relay is much decreased, and the major portion of the superimposed external excitation is actually shifted from the working airgap -- where it is predominantly needed -- to the permanent magnets for their magnetization.
It should be mentioned that electromagnetic relays are known wherein the electromagnet system includes a permanent magnet and a parallel connected shunt made of magnetically conductive material. The shunts used, however, are auxiliary shunts which are not provided for diverting large porportions of the permanent flux. They are, therefore, unsuitable for achieving a constant magnetic potential. They serve only for trimming and adjusting purposes, to compensate for variations in the properties of the components employed, manufacturing tolerances etc.